<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <input type="text" value="手机">
    <script>
        /* 需求 ： 
                1.获取焦点，如果 value 是默认值（"手机"），清空 value 字体颜色加深
                2. 失去焦点，如果是空，则添加默认值，字体颜色变淡  */

        let input = document.querySelector('input');

        input.onfocus = function () {
            if (this.value == '手机') {
                this.value = '';
            }
            this.style.color = '#333';
        }
        input.onblur = function (){
            if (input.value == '') {
                input.value = '手机'
                
            }
            input.style.color = '#999'
        }
    </script>
</body>

</html>